Ashfield Shale
Ashfield Shale is a shale formation that is part of the Wianamatta Group of sedimentary rocks in the Sydney Basin, and it lies directly on contemporaneously eroded Hawkesbury sandstone or the Mittagong formation. These rocks were formed in the Triassic Period. Ashfield Shale is generally a dark, nearly black, clay-mineral-rich rock that should be distinguished from Bringelly Shale. Ashfield Shale has outcrops over many parts of the Greater Sydney area. Many structures and deep excavations were constructed on Ashfield Shale, in addition to the shale being used as a construction material. Fossils found from the Ashfield Shale indicate freshwater lake environments existed at different eras and places.
Named after the Sydney suburb of Ashfield, some of the early research was performed at the old Ashfield Brickworks Quarry. This rock type is often associated with the Inner West and North Shore of the city. However, it has also been recorded at Penrith, Revesby, Bilpin and Mount Irvine. Ashfield Shale comprises black mudstones and grey shales with frequent sideritic clay ironstone bands. The thickness ranges between and has been subdivided into four members that comprise shifting layers of dark-coloured ferruginous shale and laminite. Ashfield Shale is associated with the critically endangered Blue Gum High Forest and Sydney Turpentine-Ironbark Forest.
Description
The Minchinbury Sandstone sits between the Ashfield Shale and the superimposed Bringelly Shale. The Minchinbury Sandstone is disconnected from the shallow water Ashfield Shale by the alluvial plain sediments of the overlying Bringelly Shale, hinting it was a strandline deposition. According to the geological classifications, Ashfield Shale can be classified as a well bound and cemented shale. The Ashfield shale was laid in a shallow marine or brackish environment. Being the basal formation of the Wianamatta group, Ashfield Shale is the only rock unit outcropping in the inner Sydney area. It has a minimum complete thickness of in the Picton area and a maximum thickness of near Erskine Park. It is thick at the Sydney Olympic Site.The geology of the shale lenses within the Hawkesbury Sandstone is chemically similar to the Ashfield Shale. The fine grained silty sediments were laid down in a low energy, south-east flowing deltaic setting, near the shores of a shallow sea. High in the shale's formation indicate a high sand content, with ripples on the formation suggesting delta front conditions. Weathering of the shale units produces a reddish/brown podsolic soil, often with poor drainage, such as that in the Cumberland Plain. These clay soils are recognised as being reactive with an appreciable shrink-swell capacity. Because of weathering, the Ashfield shale is seldom too exposed except in quarry faces. A crucial characteristic of the Ashfield Shale is its comparatively high phosphorus content that is between 0.1 and 0.9% P2O5, which tends to be sharp with salient peaks, where it resides in thin beds high in phosphatic siderite nodules.
In Lamb's Quarry, Rooty Hill, an olivine dolerite dyke intrudes Ashfield Shale. The rocks found at the Ryde-interchange site usually consist of yellow brown and dark grey shale with light grey silty laminations and lenses. At Moorebank, the Ashfield shale regularly features black to dark grey shales with laminated silty bands. At Surry Hills, they are dark grey, horizontally bedded with coarse light grey and fine grained laminations and lenses. Generally, Ashfield Shale samples had a consistent interbedded structure, characterised by frequent, very thin light-grey silty bands within dark-grey siltstone and with an average clay mineral content of 43%. Siderite is the main cementing agent, whereas mica acts as a subsidiary cementing agent. The residual soil contains significantly fewer reactive clay minerals than soils derived from Bringelly Shale. Microscopic studies of Ashfield Shale indicate that silt-sized quartz particles are embedded within a clay matrix, with widespread distribution of siderite. Having no microcracks, the shale is highly compacted, resulting in a pronounced alignment of the clay particles. It also exhibited a typical soft-rock behaviour, with a roughly linear stress–strain response up to a peak, followed by pronounced strain softening.
Lithology
The chemistry of the rock is typical of shales, with high iron levels, and some iron sulphide and low calcium levels. The Ashfield shale consists of a lower sequence of dark-grey to black, sideritic claystone and siltstone lines which grade upwardly into a fine sandstone and siltstone laminite, and then into the Minchinbury Sandstone which overlies it. The lower portion mainly features dark grey claystone and siltstone laminites which grade upwards into lighter and thicker sandstone beds. Small scale bedding is abundant. The shales are sandy at the top of the sequence. There are up to ten bands in a section. It is significantly quartzose with argillaceous and volcanic rock remnants withuncommon calcite and feldspar. Low angle crossbedding hint a littoral deposit or an marine block, possibly part of a beach and barrier system.
The porosity ranges between 5% for fresh shale from Surry Hills and Moorebank sites to around 13 % for the Ashfield shale from the Ryde-interchange site. The Kellyville laminite is characterised by a clear increase in lithic-quartz sandstone laminations. The Regentville siltstone is made up of black to dark-grey siltstone. In some places, such as south-east of Moss Vale, it progressively becomes more sandy to the top where it is overlain by the Minchinbury sandstone. The Mulgoa laminite comprises dark-grey siltstone and fine, light-grey sandstone laminae. The predominant clay minerals recorded in the shale and by earlier researchers are chiefly kaolin and illite, and calcite is relatively rare. Since there is a lack of clay minerals, the swelling in the Ashfield Shale may be due to the uptake of water between grains through the interfaces of the laminations.
Distribution
and Herbert proposed the subdivision of the Ashfield shale into four types, which from the ground upwards include: the Rouse Hill siltstone, the Kellyville laminite, the Regentville siltstone, and the topmost Mulgoa laminite. The Ashfield Shale covers about 80% of the Blacktown City Council, namely north of the Great Western Highway. Ashfield Shale outcrops extend to higher elevated areas and into places with higher rainfall than Bringelly Shale, such as the North Shore area where rainfall exceeds, the Bilpin–Mountain Lagoon area and the fringes of the Illawarra escarpment where rainfall may exceed.The stratum of Prospect quarry in Pemulwuy comprises Hawkesbury Sandstone and lacustrine Ashfield Shale. In addition to Scheyville National Park, a more striking exposure of Ashfield Shale is the river-cliff in Mulgoa Nature Reserve, just south of Penrith which presents dark coloured Mulgoa Laminite Member, and as well as the river cliff opposite of the Parramatta ferry wharf. The Mulgoa cliff can be observed from Mulgoa Road through the riparian forest, though access by foot is challenging.
Natural selection in which the Ashfield Shale is completely exposed is rare. However, it can be seen at railway and roadside cuttings, as well as old quarries. With weathering and exposure, the shale becomes a paler colour. The shale members were once partially exposed in the Thornleigh brick pit, though this has recently been filled in. Today, the most observable faces of Ashfield Shale are present along the North Shore and Northern railway lines and Bells Line of Road, and in several other road cutting exposures, like, Horace Street, St Ives and in the Picton–Campbelltown area.
Engineering and construction
The term Ashfield shale has been used to cover siltstone, claystone and laminite. In the earlier days of Sydney, the Ashfield Shale supported a number of quarries. The shale provided a suitable raw material for brickmaking. Sydney soils based on shale are not particularly fertile. But at Parramatta they proved more suitable to agriculture than those at Farm Cove, in the early days of the First Fleet. The Wianamatta group rocks, particularly the Ashfield shale, and their weathering stock are important for engineering as they shape the foundations for many buildings of Sydney CBD and Western Sydney. Ashfield shale has also been used for constructing roads. A line to the Narellan railway station is cut through the Ashfield Shale.Notwithstanding, deep excavations in Ashfield Shale can be a risky procedure with the existence of many fractures. Because the main feature of the unit is the moderately to steeply immersing continuous, planar joints that are often linked with thrust faulting, and these may at times form an unstable wedge when exposed during excavation. If not contained properly, the rock's wedge may become unstable or skid into the excavation. The Epping to Chatswood rail link involved drilling through a mixture of sandstone and highly weathered Ashfield Shale, which required advanced ground support measures like rock anchors to manage the high horizontal stresses.
The adjacent Hawkesbury Sandstone is considered a safer bedrock than the Ashfield Shale for building construction. In 2005, the construction of the Lane Cove Tunnel was affected by the collapse of an exit ramp excavation, through Ashfield Shale. Difficulties may be encountered where the Ashfield Shale interfaces with the Hawkesbury Sandstone and the Mittagong Formation. The term Ashfield shale has been used to cover siltstone, claystone and laminite.